hwrng: npcm - Fix W=1 unused variable warning

This patch fixes an unused variable warning when this driver is
built-in with CONFIG_OF=n.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/char/hw_random/npcm-rng.c

@@ -161,7 +161,7 @@ static const struct dev_pm_ops npcm_rng_pm_ops = {
 				pm_runtime_force_resume)
 };
 
-static const struct of_device_id rng_dt_id[] = {
+static const struct of_device_id rng_dt_id[] __maybe_unused = {
 	{ .compatible = "nuvoton,npcm750-rng",  },
 	{},
 };